[GunLawyer001]

I hate to be the bearer of bad news, but better to know than not know:

Since you have this in your possession, it looks like you didn't buy a Title II NFA SP89, you bought a handgun. A handgun possessed along with a shoulder stock that will fit it is something that ATF can and will prosecute as constructive possession of an unregistered short-barreled rifle, even after US v. Thompson-Center.

Installation of a pistol grip on a pistol creates an AOW, which also requires pre-registration as an NFA firearm.

I'd delete the whole thread, sell off the buttstocks and pistol grip, and enjoy your pistol.